Blocking all questions, he said: “We are appropriately assessing the claims that have been made.”

Guardian Australia has exclusively reported the “letter of concern” written by 15 doctors who have worked on Christmas Island that was sent in late November to their employer, International Health and Medical Services (IHMS). The Department of Immigration and Border Protection had a copy by 6 December.

Morrison would say nothing about the doctors’ fears that asylum seekers have been harmed by “numerous unsafe practices and gross departures from generally accepted, medical standards”.

The minister has not “received” it and will not comment on its contents until, he says, IHMS has finished its investigation. There is no date for this and no promise to act to help people doctors have identified as needing urgent care out on Christmas Island.

“The minister’s office was not provided with a copy of the letter,” Morrison said.

Morrison blamed the failings of the offshore processing model on the previous government.

“We were left with an offshore processing policy that the previous government didn’t believe in,” he said. “They had to be dragged kicking and screaming to restore it and they only acted when the political winds blew so hard against them that they decided to restore it. An act of political escapism, which came to nothing from their point of view.”

He noted that IHMS had rejected a number of the claims made in the letter.

Greens senator Sarah Hanson-Young described the letter as “a very, very serious document”.

She said the allegations in the letter represented a “very serious breach of the duty of care that the government has for these people and is no doubt putting at risk, not just the lives of refugees, but potentially the health and welfare of the people who have to work with them”.

Labor’s immigration spokesman, Richard Marles, said the letter was a “significant concern”.

“Asylum seekers in detention deserve to be treated with dignity and should have access to basic healthcare services,” he said.